Low-pressure arc discharge lamps have been used for optical presentation of information, i.e., presentation of alpha numeric signs, graphics and pictures displayed on a screen or display, respectively. Such a display consists of a matrix of picture elements, each picture element consisting of a monochrome light signal source in the case of a monochrome display. In the case of a color presentation of information, one picture element is composed on three single lamps of the primary colors red, green and blue. The desired color impression is then created physiologically by additive mixture of the three primary colors within the human eye/brain system.
There have been proposed a wide variety of flourescent lamps of such special configuration as to be applicable to such displays. For example, FIG. 1 of UK patent application No. GB 2 145 873 A, published on Apr. 3, 1985, shows one typical lamp which comprises a phosphor-coated tubular envelope of convoluted tridimensional configuration that contains a pair of electrodes and an ionizable medium. For construction of the color display, a multiplicity of the above flourescent lamps are arranged in a matrix so as to form one picture element by the combination of three lamps having the envelope coated with respective phosphors emitting the different primary colors, i.e., red, green and blue. Although the known lamps operate satisfactorily when used in some of such displays, drawbacks still exist.
Presenting information to a large audience in the open air means looking for a correspondingly larger area display which is distinctly visible not only at night but also during daylight and with sufficient optical resolution from a greater viewing distance. In the above known lamps, only the curved portion of the U-shaped envelope is presented towards the audience so that no more than approximately 20 percent of radiation is effective. The rest is dissipating, especially through the parallel legs of the U-shaped envelope which are arranged parallel to the longitudinal axis of the lamp and substantially normal or perpendicular, respectively, to the plane of fixation of a unit, said plane being also substantially normal to the viewing direction of the spectators. The surface brightness along the envelope is substantially constant, i.e., one area along the envelope does not appear brighter than another area.
Other low-pressure arc discharge flourescent lamps primarily used for general illumination are known in which the envelope includes at least two longitudinally extending leg members joined together by a transversely extending envelope portion. Examples of such lamps which are commercially available are the "Twin Tube" and "Double Twin Tube" flourescent lamps manufactured by GTE Sylvania, Danvers, Mass. Other examples are disclosed in U.S. Pat. No. 4,374,340, which issued to Bouwknegt et al on Feb. 15, 1983; U.S. Pat. No. 4,426,602, which issued to Mollet et al on Jan. 17, 1984; and U.S. Pat. No. 4,481,442, which issued to Albrecht et al on Nov. 6, 1984. Lamps described in the above-mentioned U.S. Patents allow most of the radiation to be dissipated through the longitudinally extending leg members. The surface brightness along the envelope is also substantially constant.